1. Field of the Invention
The present invention relates to a motor equipped with a dynamic pressure bearing.
2. Description of the Related Art
In a disc reading and writing apparatus, a motor equipped with a dynamic pressure fluid bearing has become in general use as a disc driving motor. This is because a dynamic pressure fluid bearing has a unique characteristic of high reliability and longevity, which is favorable for use in such an apparatus.
By the way, there has been a strong demand for a thin-model disc drive apparatus from the market and thus a thin motor has been required to be equipped therein.
To meet such a demand, there have been disclosed various kinds of thin-model disc drive apparatuses equipped with a thrust dynamic pressure bearing disposed coaxially around a radial dynamic pressure bearing in Japanese Patent Application Laid-open Publications Nos. 2001-65552 (referred to as a patent document 1, hereinafter) and 2003-180066 (referred to as a patent document 2, hereinafter), in contrast to a motor having a thrust dynamic pressure bearing disposed at the end of a shaft.
The patent document 1 discloses a dynamic pressure bearing and a motor using the same. In the motor, a sleeve is provided, at its one end, with a protrusion portion that protrudes radially therefrom. The upper surface and the lower surface of the protrusion portion, and a pair of surfaces provided in a shaft guide member, the pair of surfaces opposing respectively the upper and the lower surfaces, constitute a thrust dynamic pressure bearing, thereby balancing dynamic pressures along a shaft direction.
The patent document 1 discloses in its paragraph 0061 a motor in which a thrust dynamic pressure bearing is formed of the upper surface of a sleeve and the lower surface of a bearing holding member. In this motor, a floating position of a rotor is controlled by balancing a rotor floating pressure generated by the dynamic pressure bearing and a magnetic force generated between a rotor magnet and a stator yoke along the shaft direction.
By the way, the dynamic pressure bearing device or the motor described in the patent document 1 needs to be provided with a pair of dynamic pressure generation portions (thrust dynamic pressure bearing portions) for keeping its rotor floated in an axial direction. Specifically, one of the thrust dynamic pressure bearing portions is formed between the upper surface of a flange portion of the sleeve and its opposing surface; and the other one is formed between the lower surface of the flange portion of the sleeve and its opposing surface.
The thrust dynamic pressure generated by this dynamic pressure generation portions is mainly determined by a gap between a patterned groove formed therein and a surface opposing the groove. Therefore, the bearing device needs to be assembled in such a way that the dynamic pressure groove is precisely formed and the gap is maintained with high precision.
However, it is very difficult to process not only one surface but also the other surface of the flange portion paying much attention to degree of parallelization or the like, because the portion can be significantly deformed by a load applied thereon.
Especially, when the flange portion is thin, the deformation is more significant and, in the end, a height of such a motor can only be reduced to a limited degree.
On the other hand, in the motor having a configuration described in the patent document 2, a magnet has to be magnetized in such a way that a degree of magnetization is not varied, in order to control a floating amount of the rotor along the axis direction. However, it is extremely difficult to control a degree of magnetization at the same time as taking account of magnetic characteristic variations that occur in a sintering process of magnet production.
Moreover, since variations in dimension are particularly larger in a magnet made of a sintered material than in other members and components made of metal, a distance between the magnet and the stator yoke varies and as a result magnetic attractive force cannot be stabilized, which makes it difficult to precisely keep the rotor floated.
In addition, there exists a disadvantage in that an eddy current occurs in the stator yoke, thereby producing a rotational load.